Field
The present application relates to test methods, apparatus, and systems for evaluating the effectiveness of organic and inorganic scale inhibitors.
Related Art
Inorganic and organic scale inhibitors (also commonly referred to as dispersants) can be used for flow assurance during the production, transportation, and processing of petroleum reservoir fluids. For example, asphaltene deposition (a type of organic scaling) is an infrequent, but severe impediment during production, transportation, and processing of reservoir fluids. Unexpected precipitation and the subsequent potential for deposition of asphaltene aggregates can cause reservoir impairment, plugging of wells and flowlines, equipment fouling issues, and processing challenges for facilities. The costs associated with the precipitation and deposition of asphaltenes from reservoir fluids during production, transportation, sample handling, and processing of reservoir fluids have been estimated to be on the order of billions of dollars worldwide as described in Rogel, E., Ovalles, C. and Moir, M., “Asphaltene Stability in Crude Oils and Petroleum Materials by Solubility Profile Analysis,” Energy & Fuels, Vol. 24(8), 2010, pp. 4369-4374.
Since mechanisms of aggregation and deposition are not completely understood, prediction, prevention, and remediation techniques may be difficult to implement. One approach to minimize the possibility for asphaltene deposition is to add one or more asphaltene inhibitors to the reservoir fluid. The asphaltene inhibitor(s) act to slow the formation of asphaltene aggregates; thereby keeping asphaltenes suspended in solution. Since the aggregation behavior of asphaltenes is challenging to predict, the selection of a suitable asphaltene inhibitor that is effective for a particular reservoir fluid is often achieved through an empirical screening process, where a pre-defined library of dispersants are applied to the crude oil sample and evaluated for effectiveness by the addition of a precipitant that induces flocculation. A “good” asphaltene inhibitor will keep asphaltenes suspended and a “poor” asphaltene inhibitor will show precipitation and sedimentation. This process is commonly known as the Asphaltene Dispersion Test (ADT). The optimal solution may be a combination of several asphaltene inhibitors, which are typically in the 50-1000 ppm range. Determining a good combination is often a time consuming process that must be completed for individual oil samples because the asphaltenes vary from one oilfield to another.
One established approach for screening asphaltene inhibitors is published by the American Society for Testing and Materials (ASTM) as standard D7061, “Standard Test Method for Measuring n-Heptane Induced Phase Separation of Asphaltene-Containing Heavy Fuel Oils as Separability Number by an Optical Scanning Device”. This method evaluates aggregation and sedimentation of asphaltenes by vertically profiling a sample vial over time using optical transmission measurements. The test is used in conjunction with a commercially available device called the Turbiscan™ LAB stability analyzer, available from Formulaction SA of L'Union, France. A precipitant like n-heptane is added to a sample of crude oil plus dispersant and an optical detection system measures the transmitted and back-scattered light. Vertical scans are performed, in which a motorized stage moves the optical system along the length of the vial in 40 micron increments.
Using the traditional ASTM D7061 method, asphaltene will form and move down the vial under the influence of gravity, which results in significant waiting time for obtaining the results of the testing. For example, after a prescribed time, typically 30 minutes, a good inhibitor will slow down asphaltene sedimentation whereas a poor inhibitor will not. Using this process, batches of asphaltene dispersants are screened and appropriate concentrations selected. The serial nature of the Turbiscan™ analysis makes finding a suitable inhibitor a time consuming task.
Another test protocol for testing inorganic scale inhibitors is NACE TM 0374-2001 and NACE TM 0197-97 or their latest iteration. These protocols require that the fluid to be tested (e.g. seawater) is mixed with a scale inhibitor in a glass jar, which is then placed into a temperature-controlled chamber/bath. Then, after waiting for an amount of time which is representative or the inhibitor exposure time, the results of the reaction are checked either visually (by absorption, scattering, cloudiness, precipitation) or quantitatively (by subsampling from the test jar and using the appropriate tool for the scale of interest).